1. Field of the Invention
The present invention relates to a method and an apparatus for electronically sensing a fingerprint on a finger surface, and more particularly to a capacitive fingerprint sensor with an adjustable gain.
2. Related Art
Accurate and cost effective verification of personal identity is becoming increasingly important. Verification of personal identity can be used to prevent calling card and credit card fraud, deter theft and misuse of portable products, such as cellular phones and laptop computers, and ensure security in electronic commerce. Many methods have been proposed for electronic identification of individuals including: passwords, hardware tokens such as credit cards and ATM cards, and even portable encryption devices that combine the ideas of password and token. All of these have the problem that it is difficult or impossible to ensure that the token is in the possession of its rightful owner.
Biometric techniques rely on verifying identity by identifying a unique feature of the individual's body, such as voice, fingerprint, hand print, signature, and retina pattern. These techniques have the advantage that they move with the individual and are theoretically capable of great accuracy. However, they suffer from the drawback that in many cases, acquiring the data requires complex equipment and complex interactions with the user. Comparison of the acquired biometric data with a database of biometric data can also be quite computationally intensive, and can consequently require tremendous computational resources. Furthermore, biometric techniques such as signatures and voice recognition are subject to relatively high error rates.
Of all presently-used biometric identification techniques, fingerprints are perhaps the most appealing. Fingerprints have been accepted for 75 years as a legal means for verifying identity "beyond all reasonable doubt," and acquiring a fingerprint requires little specific behavior by the user. Considerable research has gone into the task of extracting fingerprint features and performing database comparisons. Existing technology allows the relevant features of a fingerprint to be represented in as little as 10 bytes of data, with recognition in less than 1 second, and with false acceptance and false rejection rates of 0.01%. Furthermore, the computer hardware required for recording and comparing fingerprint data can be centralized and accessed through a telecommunications network thereby allowing costs to be amortized across many transactions.
The main barrier to the widespread use of fingerprint recognition is fingerprint acquisition. There must be an acquisition device at every identification site. Existing techniques for fingerprint acquisition rely on optically imaging the fingerprint onto a light sensitive detector such as a CCD(Charged Coupled Device), to obtain an electronic image of the fingerprint. This approach has a number of problems such as high cost, great complexity, large size, poor image quality, and susceptibility of the optical system to misalignment and breakage. Optical devices are also susceptible to "spoofing," wherein a simulacrum of the fingerprint or even a photocopy of a fingerprint image is used to fool the optical sensor.
Another method for sensing fingerprints is by using a capacitive sensor, such as the sensor disclosed in U.S. Pat. No. 5,325,442, entitled, "Fingerprint Sensing Device and Recognition System Having Predetermined Electrode Activation," by inventor Alan G. Knapp (the "Knapp patent"). The Knapp patent discloses a fingerprint sensing device comprising a planar array of closely-space capacitive sense elements. When a finger is placed in close proximity to the sensing device, the capacitive sense elements measure a capacitance between the finger surface and single capacitor plate in each sense element. This is accomplished by sensing a drive current while driving a voltage onto the capacitor plate. The measured capacitance varies as a function of the distance between the capacitor plate and the finger surface. Thus, a capacitance measurement allows the distance between the capacitor plate and the finger surface to be determined. Distance measurements across the array of sense elements are combined to produce a representation of the pattern of ridges on the finger surface which comprise a fingerprint.
There are however a number of serious flaws with the design disclosed in the Knapp patent. (1) The Knapp invention operates by driving a charging current onto a column line which feeds into a sense electrode and measuring a charging current to determine a capacitance between the sense electrode and the finger surface. In such a configuration, because of the length of the column line and the small size of the capacitance being measured, discriminating between the charging of the column line and the charging of the electrode plate is a difficult if not impossible task. (2) No provision is made in the design for variations in capacitance due to, for example, dry finger surfaces and temperature variations. (3) Additionally, the invention disclosed in the Knapp patent measures a charging current. This charging current is likely to vary widely over time as the electrode plate and the attached column line are charged. Hence, it is unlikely that a charge current measured at a particular instant in time will be an accurate indicator of capacitance.
What is needed is a capacitive fingerprint sensor with sufficient sensitivity to gather a fingerprint from a finger surface, and with sufficient adjustability to deal with variations in capacitance due to changes in moisture and temperature characteristics of the finger surface.